Due to its advantages of long cycle life, high safety performance, low cost, good temperature stability, abundant resources, etc., the lithium iron phosphate (LiFePO4) material becomes the first choice of cathode material for a lithium ion power battery. Particularly, the lithium iron phosphate material draws the attention of many domestic and overseas research and development organizations and manufacturers.
However, the lithium iron phosphate material has a defect of low electrical conductivity, which results in high electrochemical polarization and resistance polarization. At present, major methods adopted home and abroad for solving the problem refer to carbon coating, which lowers the contact resistance among lithium iron phosphate particles by the high conductivity of carbon so as to realize the aims of reducing polarization and prolonging cycle life. For example, a Chinese Patent of Invention with the Publication Number of CN101154722A and the Invention Title of “Nuclear shell nanoscale carbon-coated lithium iron phosphate composite cathode material and preparation method thereof”, a Chinese Patent of Invention with the Publication Number of CN101162776A and the Invention Title of “Lithium iron phosphate suitable for high magnification power battery and preparation method thereof”, a Chinese Patent of Invention with the Publication Number of CN101172599A and the Invention Title of “Method for preparing carbon-coated lithium iron phosphate”, a Chinese Patent of Invention with the Publication Number of CN101212049A and the Invention Title of “Cathode material consisting of doped lithium iron phosphate active substance and carbon”, etc. are carbon coating methods. Coated carbon material is generally formed through cracking of carbon-containing organic substances such as cane sugar, glucose and cyclodextrin while sintering during the producing process of lithium iron phosphate material.
Some technology adopts a method for doping heterogeneous ions and improving the conductivity of a lithium iron phosphate substrate, such as a Chinese Patent of Invention with the Publication Number of CN1773754A and the Invention Title of “Cathode material consisting of lithium iron phosphate and composite metal phosphide thereof and preparation method”, a Chinese Patent of Invention with the Publication Number of CN1785799A and the Invention Title of “Method for preparing transition element-doped lithium iron phosphate powder” and a Chinese Patent of Invention with the Publication Number of CN1830764A and the Invention Title of “Rare earth-doped carbon-coated nanometer cathode material lithium iron phosphate and preparation method thereof”.
A Chinese Patent of Invention with the Publication Number of CN101339988A and the Invention Title of “Cathode material of lithium ion battery and preparation method thereof” also discloses a method for generating metal copper on the surface of a lithium iron phosphate material by soaking with copper nitrate and reducing with VC. A Chinese Patent of Invention with the Publication Number of CN1649189A and the Invention Title of “Method for preparing carbon-coated lithium ferrous phosphate composite material containing metal conductive agent” discloses a method for forming elemental copper or silver on the surface of the material by high temperature reduction of copper or silver salt. The technology can improve the electrochemical performance of the material to a certain extent. However, some problems also exist. For example, carbon will severely reduce the stacking and compacted density of the material and large carbon agglomerates tend to form when dispersion is poor. The effect of heterogeneous ion doping is unstable and its function is also unstable. The conductive metal reduction method has complex operation process and it is very difficult to ensure that metal does not undergo side reactions such as oxidation in subsequent processing process.